Vertical venetian blinds have come into considerable prominence in recent years, both as a substitute for the more common horizontal type, and for use in installations which are wholly unsuited for horizontal type blinds. One relatively expensive construction employs a horizontally oriented worm which drives a separate pinion on each slat to simultaneously vary the adjusted angle of the slat between open and closed positions. A construction of substantially lower cost involves the suspending of each slat at an upper end thereof by a clamp member having an upstanding pintle terminating in a hemispherically shaped member which rides upon the edges of a horizontally oriented slot in a supporting element fixed to the upper horizontal portion of a window frame, wall or ceiling. These slats may be drawn by pulling ball chains linking the slats together, and rotation about a vertical axis by pulling other ball chains linking these slats, depending upon the desired direction of rotation. In constructions of this type, because of the difficulty of meeting close commercial tolerances, equal angular rotation of each slat with a pair of chains is not readily obtained, resulting in the rotation of the slats to relatively closed overlapped relation in an uneven manner, with accompanying unsatisfactory appearance and light control.